1. Field of the Invention
This invention relates to heat treatable hardfacings. In particular, this invention entails a molding/sheathing/compaction process for making high-density powder composite rods for weld-applied hard facing.
2. Description of the Related Art
Hard metal overlays are employed in rock drilling bits and other downhole tools as wear and deformation resistant cutting edges and faying surfaces. These comprise composite structures of hard particles in a metal matrix. Such hard metal overlays normally are formed by brazing or weld deposition of composite rod, producing a metal alloy matrix solidified from a melt containing hard particles that remain at least partially solid. Filler metals comprising composite rod containing both matrix and hard particle constituents has been in use for several decades. Early examples of hardfacing rods for welding are shown in U.S. Pat. Nos. 1,757, 601 and 3,023,130.
Hard metal overlays used on steel-toothed rolling cutter drill bits are subjected to extreme loads and prolonged scraping action. Therefore, the strongest, most wear resistant of fused hard metals are used in these cutting structures. Typically, such hard metal composites utilize sintered pellets or grains of cemented tungsten carbide/cobalt as the primary hard phase.
In addition to steel tooth rolling cutter drill bits, other types of down-hole tools also benefit from a strong wear resistant hardfacing material. For example, fixed cutter type earth boring drill bits and stabilizers often utilize welded hardsurfacing to protect gauge, blade, or watercourse surfaces. In a relatively recent development, tools made to steer drill bits during the drilling operation provide among the most demanding applications for hard facing materials.
The formulation of composite rod filler metal entails fabrication and process considerations in addition to constituency selection. Typically, a tubular construction has been employed wherein a metal sheath is formed to enclose a particulate mixture comprising hard particles phases and additives including binders and de-oxidizers. In such rod, the sheath metal combines with substrate melt, if any, to provide substantially all of the matrix phase of the final composite. The constituency of the hardmetal deposit is dependent on deposition process parameters as well as on raw material formulation.
The management of thermal inputs during weld deposition is critical to deposit soundness and performance. Insufficient substrate heating and/or insufficient filler metal superheat can cause poor bonding, porosity, and irregular deposit configuration. Excess substrate heating, and/or excess filler metal superheat, and/or prolonged molten time produces substrate dilution and hard-particle degradation. Substrate dilution reduces carbide fractions, while sintered particle degradation causes softening and matrix embrittlement.
As the carbide loading and application surface areas increase, weld temperature and time control become increasingly critical. Composite rod with more than about 60 weight % carbide fill is problematic to weld without substrate penetration and dilution, especially on large substrates. Deposition dynamics are strongly influenced by thermal transfer, melting, and flow characteristics of the composite rod.
Melting dynamics can be accelerated by incorporating metal matrix components of the composite rod as a powder rather than as a solid sheath. This approach exploits the high specific surface area of particulate material to speed up melting, while eliminating transport and mixing dynamics. However, sheath elimination also entails loss of its structural contributions to handling strength and melt progression. U.S. Pat. Nos. 4,836,307; 4,944,774; and 5,051,112 (all incorporated by reference herein for all they disclose) disclose the sintering of a powdered composite rod as a means of replacing the mechanical strength of the sheath. Such sintered pre-forms develop a strong, porous structure which acts to impede heat flow prior to melt collapse into the weld pool. As a result, some melting speed is sacrificed and melt progression becomes more difficult to control, resulting in operator-induced thickness and composition variation in the hard facing. In U.S. Pat. No. 4,699,848, a wire-reinforced powder rod is disclosed, replacing external sheath with solid metal filler at the center of the rod, the location most limited by thermal flow from external heat sources. This construction exacerbates weldability and control limitations, compared with conventional practice.
In U.S. Pat. No. 5,740,872 (incorporated by reference herein for all it discloses) a powder composite rod is disclosed with a thin metal sheath wherein the ratio of powder metal to sheath metal exceeds 2.5. The fabrication of such methylcellulose-bound powder metallurgy composite rod for weld-deposited hard surfacing as described in this patent has been conducted by extrusion and desiccation of rod cores, followed by sheath attachment using a wrapping mill. The rod core produced in this process has a void volume of about 40 vol %, relying on the binder for green handling strength. The sheath is wrapped with a simple overlap and attached to the core by a silicate adhesive that partially infiltrates the porous core, providing additional handling strength and preventing core movement within the sheath. The silicate adhesive becomes a liquid slag during weld application, that must be manipulated out of the deposit, slowing application rates and demanding greater operator skill. These silicate influences all lend to adverse deposit effects, including pellet degradation, porosity, inclusions, and reduced thickness control. Although the thin sheath extruded rod filler metal and application process provide net improvements in application productivity, quality, and in the hard facing performance as compared with conventional practice, its utility is limited by silicate adhesive effects and also by the relative brittleness of low-density methylcellulose-bound powder cores.
The present invention is a method for forming a high-density composite rod for hard facing. The method includes the steps of: preparing a powder mix comprising carbide and metal powders, a fugitive binder, and other additives to render a moldable rheology; forming the powder mix into a powder core encased with a metal sheath to form a core-sheath assembly; and isostatically compacting the core-sheath assembly to densify the core to at least 65% of theoretical density and mechanically attach the sheath. The core fugitive binder along with residual volatile constituents may be thermally removed with or without vacuum, reducing, or inert environment prior to weld application of the composite rod.
It is contemplated that the method has application to downhole tools including both fixed cutter and rolling cutter drill bits, bias pads for downhole rotary steerable systems, stabilizers, and other tools requiring strong and wear resistant hardfacings.